When the Nexus 4 was officially launched, it sold out quickly. A couple weeks later, Google re-opened the orders for another go. It went a little bit smoother the second time, but users were left waiting anywhere between two to eight weeks to get their glorious N4s. During all of this, though, the official bumper cases were sold out basically the entire time. Some users were able to grab one during the initial order process, but that's the extent of it - they've been out of stock ever since.
They're finally back now (on the U.S. Play Store, anyway. Something tells me they'll be gone again shortly, so you better hurry and jump on it if you plan on picking one up.
Thanks, Calvin!